The step from diagenesis to metamorphism in pelitic rocks during alpine orogenesis

TLDR: In this article, a sequence of irregular mixed-layer illite/montmorillonite was proposed for the formation of paragonite, and the following sequence was proposed: irregular mixed layer illite, regular mixed layer mica, mixed layer phengite/phengite.

Abstract: Summary Pelitic rocks were followed from unmetamorphosed clays and marls of Upper Triassic and Lower Liassic to anchimetamorphosed phyllites of the Alpine border region in Switzerland. Phengite and Al-rich chlorite formed from mixed-layer illite/montmorillonite; pyrophyllite from kaolinite. For the formation of paragonite, the following sequence is proposed: irregular mixed-layer illite/montmorillonite regular mixed-layer mica/montmorillonite mixed-layer paragonite/phengite paragonite. Besides the formation of new minerals in the transition zone (anchizone) between diagenesis and greenschist facies, other changes with increasing metamorphic grade are: the crystallinity of illite (in the sense of Kubler, 1967) increases, the intensity ratio 002/001 of the illite basal reflexions increases, 1Md illite changes to 2M1 phengite; the slates change colour from red to pink; the mean density increases; and textural changes due to reactions between clastic quartz and clay cement occur.